1. Field of the Invention
The present invention relates to a luminance-enhancing film, an optical sheet member, and a liquid crystal display device. More specifically, the present invention relates to a luminance-enhancing film which has high luminance and is able to suppress an oblique color change at the time of being incorporated in a liquid crystal display device, an optical sheet member using the luminance-enhancing film, and a liquid crystal display device using the luminance-enhancing film.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter, also referred to as LCD) has been annually variously used as a space saving image display device having low power consumption. The liquid crystal display device, for example, is configured by disposing backlight (hereinafter, also referred to as BL), a backlight side polarizing plate, a liquid crystal cell, a visible side polarizing plate, and the like in this order.
In the recent flat panel display market, power saving, high definition, and enhancement in color reproducibility have progressed as enhancement in LCD performance. In particular, the progress of the enhancement in LCD performance is remarkable in a small-size liquid crystal display device of a tablet PC, a smart phone, or the like.
On the other hand, a next-generation hi-vision (4K2K, an EBU ratio of greater than or equal to 100%) of the current TV standard (FHD, a national television system committee (NTSC)) ratio of 72%≅an European broadcasting union (EBU) ratio of 100%) has been developed in a large-size liquid crystal display device used for TV, and development for power saving, high definition, and enhancement in color reproducibility has progressed as performance enhancement such as a reduction in size. For this reason, in the liquid crystal display device, power saving, high definition, and enhancement in color reproducibility have been increasingly required.
It has been proposed that a reflection polarizer is disposed between the backlight and the backlight side polarizing plate according to power saving of the backlight. The reflection polarizer is an optical element in which among incident light rays while vibrating in all directions, only light rays vibrating in a specific polarization direction are transmitted, and light rays vibrating in the other polarization direction are reflected. Accordingly, it is possible to recycle the light rays which are reflected without being transmitted through the reflection polarizer, and it is possible to enhance light utilization efficiency in the LCD.
In response, a technology has been known in which an optical sheet member (a dual brightness enhancement film (DBEF: Registered Trademark) or the like) is combined between the backlight and the backlight side polarizing plate, and thus a light utilization rate of the BL is improved, and the luminance is improved while saving power of the backlight (refer to JP3448626B).
In JP1989-133003A (JP-H01-133003A), a technology is disclosed in which a reflection wavelength region is broadened by a reflection polarizing plate configured by laminating a λ/4 plate and a layer formed by fixing a cholesteric liquid crystalline phase and three or more layers formed by fixing cholesteric liquid crystalline phases having different pitches, and thus a light utilization rate of the BL is enhanced.
Here, when the reflection polarizing plate configured by laminating the λ/4 plate and the layer formed by fixing the cholesteric liquid crystalline phase is incorporated in the liquid crystal display device, it has been known that a change in the shade when viewed from an oblique direction which is caused by optical properties of the cholesteric liquid crystalline phase and the λ/4 plate easily occurs. In response, in JP3518660B, a method in which the pitch of the cholesteric liquid crystalline phase is shortened at an incidence side of the light, and disposing a compensation layer having a refractive index in a vertical direction which is greater than an in-plane refractive index are proposed. In addition, in WO2008/016056A, a method is proposed in which retardation of the λ/4 plate in a thickness direction is set to be less than 0.
Furthermore, when the layer is formed by fixing the cholesteric liquid crystalline phase, in general, a rod-like liquid crystal compound is used as a cholesteric liquid crystal material, and the same reflection function as that of the cholesteric liquid crystalline phase using the rod-like liquid crystal compound is able to be obtained by aligning a discotic liquid crystal in the shape of a spiral. The discotic liquid crystal having a spiral structure, for example, is disclosed in JP2001-81465A.
In addition, a method is proposed in which a plurality of layers having different pitches are disposed or the pitch is gradually changed as a polarizing plate using a layer formed by fixing the other cholesteric liquid crystalline phase in order to broaden the reflection range.
On the other hand, a method has been also known in which a light emitting spectrum of the backlight becomes sharp from a viewpoint of high definition and enhancement in color reproducibility in the liquid crystal display device. For example, in JP2012-169271A, a method is disclosed in which white light is embodied by using a quantum dot emitting red light and green light between a blue LED and a light guide plate as a fluorescent body, and thus high luminance and enhancement in color reproducibility are realized. In SID'12 DIGEST p. 895, a method of combining a light conversion sheet (QDEF, also referred to as a quantum dot sheet) using a quantum dot for enhancing color reproducibility of the LCD is proposed.